/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.server.i18n;

import java.util.ArrayList;
import java.util.List;
import java.util.Locale;

import org.springframework.context.i18n.LocaleContext;
import org.springframework.context.i18n.SimpleLocaleContext;
import org.springframework.http.HttpHeaders;
import org.springframework.lang.Nullable;
import org.springframework.util.CollectionUtils;
import org.springframework.util.StringUtils;
import org.springframework.web.server.ServerWebExchange;

/**
 * {@link LocaleContextResolver} implementation that simply uses the primary locale
 * specified in the "Accept-Language" header of the HTTP request (that is,
 * the locale sent by the client browser, normally that of the client's OS).
 *
 * <p>Note: Does not support {@link #setLocaleContext}, since the accept header
 * can only be changed through changing the client's locale settings.
 *
 * @author Sebastien Deleuze
 * @author Juergen Hoeller
 * @since 5.0
 */
public class AcceptHeaderLocaleContextResolver implements LocaleContextResolver {

    private final List<Locale> supportedLocales = new ArrayList<>(4);

    @Nullable
    private Locale defaultLocale;


    /**
     * Configure supported locales to check against the requested locales
     * determined via {@link HttpHeaders#getAcceptLanguageAsLocales()}.
     *
     * @param locales the supported locales
     */
    public void setSupportedLocales(List<Locale> locales) {
        this.supportedLocales.clear();
        this.supportedLocales.addAll(locales);
    }

    /**
     * Return the configured list of supported locales.
     */
    public List<Locale> getSupportedLocales() {
        return this.supportedLocales;
    }

    /**
     * Configure a fixed default locale to fall back on if the request does not
     * have an "Accept-Language" header (not set by default).
     *
     * @param defaultLocale the default locale to use
     */
    public void setDefaultLocale(@Nullable Locale defaultLocale) {
        this.defaultLocale = defaultLocale;
    }

    /**
     * The configured default locale, if any.
     */
    @Nullable
    public Locale getDefaultLocale() {
        return this.defaultLocale;
    }


    @Override
    public LocaleContext resolveLocaleContext(ServerWebExchange exchange) {
        List<Locale> requestLocales = null;
        try {
            requestLocales = exchange.getRequest().getHeaders().getAcceptLanguageAsLocales();
        } catch (IllegalArgumentException ex) {
            // Invalid Accept-Language header: treat as empty for matching purposes
        }
        return new SimpleLocaleContext(resolveSupportedLocale(requestLocales));
    }

    @Nullable
    private Locale resolveSupportedLocale(@Nullable List<Locale> requestLocales) {
        if (CollectionUtils.isEmpty(requestLocales)) {
            return this.defaultLocale;  // may be null
        }
        List<Locale> supported = getSupportedLocales();
        if (supported.isEmpty()) {
            return requestLocales.get(0);  // never null
        }

        Locale languageMatch = null;
        for (Locale locale : requestLocales) {
            if (supported.contains(locale)) {
                // Full match: typically language + country
                return locale;
            } else if (languageMatch == null) {
                // Let's try to find a language-only match as a fallback
                for (Locale candidate : supported) {
                    if (!StringUtils.hasLength(candidate.getCountry()) &&
                            candidate.getLanguage().equals(locale.getLanguage())) {
                        languageMatch = candidate;
                    }
                }
            }
        }
        if (languageMatch != null) {
            return languageMatch;
        }

        return (this.defaultLocale != null ? this.defaultLocale : requestLocales.get(0));
    }

    @Override
    public void setLocaleContext(ServerWebExchange exchange, @Nullable LocaleContext locale) {
        throw new UnsupportedOperationException(
                "Cannot change HTTP accept header - use a different locale context resolution strategy");
    }

}
